1. Field of the Invention
The present invention relates to a method and apparatus for gauging and rounding a section of pipe, and more specifically to gauging and rounding a section of fully formed seamless ductile iron pipe at room temperature.
2. Description of Related Art
Ductile cast iron pipe of the bell-and-spigot type includes an enlarged or bell end and an opposite end designed to be inserted into the bell end of an adjoining pipe. In the production of ductile cast iron pipe, it has been known in that art that the casting, solidification, and rapid cooling steps during the manufacturing process result in undesirable carbides and pearlite forming in the iron pipe. As a result, the pipe is normally given an annealing heat treatment to form a ferritic microstructure with graphite nodules. This microstructure has much improved properties with respect to ductility, elongation, and toughness when compared to the as-cast properties. However, the result of the heat treating process and the conversion of the microstructure results in dimensional changes in the casting. The pipe grows in diameter and length as a result of the annealing process. This growth leads to much difficulty in controlling the pipe diameter within the desired dimensional tolerances and the degree of roundness of the pipe. This is especially a problem for bell-and-spigot type pipe, which necessarily involves mating successively joined pipes. For example, 24-inch diameter cast iron pipe sections can be as much as 2 inches out of round. Larger diameter pipe sections can be 5 to 10 inches out of round. Such a degree of out-of-roundness makes such pipe sections virtually unusable in pipelines using push-on bell-and-spigot joint connections.
It has commonly been agreed or accepted in the ductile iron pipe art that, because the iron is in cast form, the material will generally not respond favorably to cold working to change its shape or dimensions. Cold working of cast iron usually results in the formation of cracks that render the cast article useless. As a result, persons skilled in the art would not generally attempt to round or gauge pipe sections that required any substantial amount of deformation, and especially not cold deformation, and prior processes and apparatus were geared toward making only minor corrections in the roundness or gauge of the pipe.
It is therefore a principal object of the present invention to provide a method and apparatus for gauging and rounding a section of fully formed centrifugally cast ductile iron pipe at room temperature from an initial out-of-round cross-section profile and initial gauge to a final, substantially circular cross-section profile and final gauge, without sustaining cracking or other failure modes.
It is a further important object of the present invention to achieve a final gauge of the ductile cast iron pipe that is smaller than the initial gauge of the pipe.
It is another important object of the present invention to achieve a substantially smoothing of the cast peen pattern on the outer dimension surface of the pipe that is suitable for uniform application of surface coatings.
It is an additional important object of the present invention to provide the above-discussed advantages in a relatively economical design.
The above and other objects of the present invention are obtained by providing an apparatus and method for gauging and rounding a section of fully formed seamless ductile iron pipe that has an out-of-round initial cross-section profile and/or a larger than desired initial gauge. This is accomplished through the use of a set of gauging/rounding rolls, each roll having an hour-glass shape, whereby the set of rolls is arranged so that the combined profiles of adjacent rolls forms a substantially continuous circle of a precise diameter. The exact diameter of the circle formed by the hourglass-shaped rolls is determined by the dimensions required for the final outside diameter of the ductile iron pipe, taking into consideration the elastic unloading of the pipe when it is removed from the apparatus.
In the specification, and especially in the claims, the term xe2x80x9cplastically deformingxe2x80x9d will be employed to cover the gauging and/or rounding of a pipe section. This is because there may be instances in which gauging or rounding is not actually effected, and the process is not to be limited to a process in which both are always performed and achieved.
In operation, the ductile iron pipe is passed through the circle of gauging/rounding rolls, with the pipe being at substantially room temperature, or at some elevated temperature, at the start of the process. The rounding/gauging of the pipe is accomplished through compressive yielding of the ductile iron in the hoop or tangential direction, and in the radial direction, to accommodate and conform to the dimension and roundness of the hourglass rolls.
Advantageously, in the course of performing the rounding and/or gauging, the normally heavy peen pattern on the outer surface of the pipe, imparted by the mold surface in the centrifugal casting process, is substantially smoothed by the compressive forces exerted by the rolls and the attendant yield of the material. Such heavy peen patterns are difficult to coat due to the surface roughness. Passage of the pipe through the gauging/rounding rolls reduces the height of the peens, fins, and other asperities on the pipe outer surface.
A transport mechanism for conveying the pipe through the gauging/rounding roll arrangement consists of a rail mounted push car which is moved by one or more fluid actuated cylinders anchored to a base in common with the rail. Optionally, a pair of such cars, each engaged with the pipe, operate to move the pipe into, and out of, the gauging/rounding roll arrangement.